This invention relates to a battery and, more particularly, to a battery structure that realizes a light and thin battery having a high discharging current at a high current density and satisfactory cycle characteristics.
Batteries have been used long as a main power source or a backup power source for a variety of equipment. The demand for batteries has recently been increasing with the development of portable electronic equipment, such as cellular phones and portable personal computers. Primary batteries and secondary batteries are available according to use. As to secondary batteries having great convenience, high performance batteries such as lithium ion secondary batteries and nickel-hydrogen batteries have been attracting attention. The present invention will hereinafter be explained by referring to lithium ion secondary batteries the demand of which has been steeply increasing for use in portable electronic equipment.
Conventional lithium ion secondary batteries comprise a battery body that is a cylindrical roll of an electrode body or a stack of rectangular electrode bodies, the electrode body being composed of a positive electrode, a negative electrode, and a separator that is interposed between the two electrodes to serve for insulation and retention of an electrolyte. The battery body is put in a metal-made case so that the positive electrode, the negative electrode and the separator can be brought into intimate contact by the pressure of the case thereby to maintain the contact between each electrode and the separator.
An electrical contact can be maintained by putting the battery body in a metal-made case, but there is a problem that the case, being made of metal, increases the weight of the battery. Moreover, it is difficult to make a thin metal case. Difficulty in making a thin case has been a great obstacle to fulfillment of the demand for batteries to be used in compact portable equipment.
In this connection, U.S. Pat. No. 5,437,692 discloses a structure in which a lithium ion-conducting polymer is used as an ion conducting layer, and a positive electrode and a negative electrode are joined to the ion-conducting layer with an adhesive layer containing a lithium compound. The inventors of the present invention previously proposed in Japanese Patent Application No. 8-338240 a battery structure requiring no metal-made rigid case and a process for producing the same, in which a positive electrode and a negative electrode are previously joined to a separator with an adhesive resin.
Bonding positive and negative electrodes to a separator with an adhesive resin has made it feasible to maintain an electrical contact among them without imposing an external force. However, being insulating, in nature, an adhesive resin present in the interface between a positive and a negative electrode and a separator tends to shut an electrical flow, i.e., ion conduction.
In bonding a positive and a negative electrode to a separator with an adhesive resin, the adhesive strength tends to increase with the amount of the adhesive resin in the interface. There is a tendency, however, that battery characteristics are deteriorated with an increasing amount of the adhesive resin. That is, conflict between adhesive strength and battery characteristics is observed. As the amount of the adhesive resin increases, the adhesive area tends to increase because the spots of the adhesive resin applied to the interface increase ultimately to form a film covering the interface. As a result, the adhesive strength increases, but, with the interface between electrodes being covered with an insulating film, it seems that ion conducting passages between electrodes are reduced, resulting in deterioration of the battery characteristics. Where, on the other hand, the adhesive resin concentration in a solution type adhesive for bonding is diminished for the purpose of improving battery characteristics, the adhesive resin solution having a reduced viscosity penetrates into the electrodes that are porous only to exhibit low adhesive strength or even fail to bond. It has therefore been a significant theme to improve battery characteristics while retaining adhesive strength.
Electrodes have their surfaces smoothed by pressing but still have unevenness of several microns to form vacancies where a separator and the electrodes are not in contact. The vacancies that should have been filled with an electrolyte may get starved of the electrolyte, which depends on the amount of the electrolyte supplied and the condition of use of the battery. Starvation of the electrolyte leads to an increase of internal resistivity of the battery and reductions in battery characteristics.
The present invention has been reached, aiming at settlement of the above-described problems. It is an object of the invention to provide a light and thin battery which has improved battery characteristics while securing adhesive strength.
A first battery according to the invention comprises a battery body having a positive and a negative electrode containing an active material, a separator holding an electrolyte, and an adhesive resin layer joining the positive and the negative electrodes to the separator, wherein the adhesive resin layer is composed of at least one layer and contains a filler. According to this structure, the filler added makes the adhesive resin layer porous. The electrolyte and the adhesive resin solution can be held in the pores so that satisfactory battery characteristics can be obtained while securing adhesive strength.
A second battery according to the invention is the above-described first battery, wherein the electrolyte is an organic electrolyte containing lithium ions. This mode, when applied to lithium ion secondary batteries which are required to have reduced weight and thickness, provides a high performance compact battery.
A third battery according to the invention is the above-described first battery, wherein the average particle size of the filler is equal to or smaller than the particle size of the active material of the positive and negative electrodes. According to this mode, the adhesive resin solution is held by the adhesive resin layer to give necessary adhesive strength.
A fourth battery according to the invention is the above-described first battery, wherein the average particle size of the filler is 1 xcexcm or smaller. According to this embodiment, the filler manifests a proper thickening effect for the adhesive resin solution and makes the adhesive resin layer porous thereby to secure satisfactory battery characteristics while retaining adhesive strength.
A fifth battery according to the invention is the above-described first battery, wherein the sum of a volume ratio of the adhesive resin and that of the filler per unit volume of the adhesive resin layer is less than 1. This mode secures the porosity of the formed adhesive resin layer.
A sixth battery according to the invention is the above-described first battery, wherein the sum of a volume ratio of the adhesive resin and that of the filler per unit volume of the adhesive resin layer is 0.2 to 0.8. According to this embodiment, the voids of the porous adhesive resin are filled with the electrolyte to exhibit sufficient ion conductivity.
A seventh battery according to the invention is the above-described first battery, wherein the filler comprises at least one of non-conductive materials and semiconductors. According to this mode, the adhesive resin layer can be made porous to provide satisfactory battery characteristics while retaining adhesive strength.
An eighth battery according to the invention is the above-described first battery, wherein the adhesive resin layer comprises a layer containing an electrically conductive filler and a layer containing at least one of non-conductive materials and semiconductors. According to this embodiment, the conductive filler-containing layer functions to diminish the internal resistivity of the battery.
A ninth battery according to the invention is the above-described first battery, wherein the adhesive resin layer is constituted so as to fill the vacancies formed in the interface between each electrode and the separator due to the unevenness of the electrode and the separator. This structure is effective in increasing the adhesive strength and preventing reduction of battery characteristics due to starvation of the electrolyte.
A tenth battery according to the invention is the above-described first battery, wherein the battery body is a laminate of a plurality of electrode bodies each composed of a single layer of the positive electrode, a single layer of the separator, and a single layer of the negative electrode.
An eleventh battery according to the invention is the above-described tenth battery, wherein the laminate is formed by interposing the positive electrode and the negative electrode alternately among a plurality of the separators.
A twelfth battery according to the invention is the above-described tenth battery, wherein the laminate is formed by interposing the positive electrode and the negative electrode alternately between rolled separators.
A thirteenth battery according to the invention is the above-described tenth battery, wherein the laminate is formed by interposing the positive electrode and the negative electrode alternately between folded separators.
The tenth to thirteenth embodiments are effective in providing a laminated electrode type battery having high performance and a high battery capacity.